7. Project Summary What enables a baby's brain to learn so rapidly during early developmental critical periods? Is it possible to re- engage in adult the enhanced learning capacity present in the child's brain for repair or restoration of function? We have discovered that a neuronal receptor called PirB (Paired Immunoglobulin-like receptor B; human LilrB2/3) regulates synapse pruning and plasticity in cerebral cortex. Blocking PirB function in visual cortex rapidly leads to new dendritic spines and functional synapses even in adult. These observations have clinical relevance. A hallmark of Alzheimer's disease (AD) is loss of plasticity and excessive synapse pruning. But when PirB is knocked out in a mouse model of AD, mice are protected from memory loss. Moreover, new synapses generated by PirB blockade can be captured to facilitate recovery from severe vision loss in Amblyopia. A major goal of this research proposal is to save or regenerate synapses by understanding cell and molecular mechanisms of how PirB regulates synapse pruning. Three specific aims are proposed. 1) Assess effects of PirB deletion on dendritic spine density and stability in visual cortex. We have generated mice with a conditional allele of PirB (PirB flox/flox). We have also made a soluble PirB function- blocking recombinant protein: sPirB. These mice and reagents permit cell-type specific and temporal disruption of PirB and will be used in 2 photon imaging and physiological studies of synapse pruning and spine turnover in cortical pyramidal neurons. Results from these experiments should illuminate further how PirB regulates plasticity and synapse pruning in juvenile and adult visual cortex. 2) Investigate how PirB functions by using expression profiling to identify PirB signaling pathways. Preliminary experiments suggest that cofilin signaling contributes to PirB dependent regulation of dendritic spine density, and is hyperactivated in a mouse model of AD. Here an unbiased approach to identify PirB-driven alterations in mRNA expression is proposed. RiboTag mice will be used to isolate neuronal transcripts from PirB+/+ vs PirB-/- visual cortex. Using microarrays and RNAseq, the transcriptomes of the 2 genotypes will be analyzed and compared. This approach should facilitate identification of candidate signaling pathways regulated by PirB. 3) PirB and Alzheimer's disease- Engineer high affinity, specific blockers of the PirB-Abeta interaction. We have measured a direct interaction between PirB and soluble oligomers of beta amyloid (Abeta), the pathogenic 42 amino acid peptide in AD. The PirB-Abeta interaction represents a novel and potentially pivotal signaling axis. Protein engineering techniques including yeast display and affinity maturation will be used to create molecular entities able to block the Abeta-PirB (or Abeta-LilrB2 in human) interaction, potentially yielding new drug targets to treat AD. In sum, this proposal seeks to extend understanding of PirB function and dysfunction towards the ultimate goal of designing novel therapies that can regenerate lost synapses, thereby restoring function to neural circuits whose connections have been altered by abnormal experience or disease.